The invention relates to a device for moving and guiding a motion-picture film with sprocket holes through the film gate of a telecine scanner.
When motion-picture films are scanned synchronisation errors arise, for example, as a result of transport fluctuations, dynamic variations in film traction owing to the elasticity of the film stock and, in particular, frame misalignment on the film, which errors may lead to visible horizontal and vertical image unsteadiness. Whereas image unsteadiness may not be annoying in the case of projection in darkened movie theatres for lack of suitable reference points, the annoying effect may increase considerably during television reproduction owing to the illuminated environment and the rapid alternation of electronic recorded images and film shots. Very stringent requirements are imposed on the image steadiness in the case of processes in which computer graphics are combined with film shots by means of chroma-key techniques because the relative movement of the inserted film scenes in the stationary part of the image is particularly annoying.
To mitigate these drawbacks German Utility Model G 93 07 566 proposes a device for the transport of a film through the scanning gate of a film scanner. The '566 device includes a carriage which is rectilinearly reciprocated in a guide perpendicular to the optical path of the film gate and which causes two transport pins which are engageable in reference sprocket holes of the film and define the position of each film frame. The pins immobilize the film frame on the carriage during movement of the carriage. This construction provides a stable positioning of the film frame to be scanned on the reciprocating slide. However, this known device has the drawback that the simple eccentric drive used for the slide movement only allows a sinusoidal movement without any real rest positions during slide reversal, so that owing to the continuous movement of the film the transport cycle may be disturbed again both when the registration pin engages and disengages the sprocket holes. Moreover, this is liable to give rise to damage to the sprocket holes.
Therefore, it is an object of the present invention to provide a device of the type defined in the opening paragraph, which mitigates the above-mentioned drawbacks of the known device and which gives an optimum image steadiness during scanning of the film frame with the normal film speed.
This object is achieved in that drive means for the carriage comprises a rotatable cam having a sawtooth-shape with sinusoidal transitions and two latching positions, the sawtooth-shape having a linearly rising portion (a) of the sawtooth to generate a constant driving speed corresponding to the real film speed, the sinusoidal transitions serve to generate an approach phase and a running-out phase, and one of the two latching positions, which serve to interrupt the carriage movement, precedes and the other follows the linearly rising portion (a).
The device in accordance with the invention having the characteristic has the advantage that owing to the specially shaped cam the carnage performs a so-called pilgrim-step movement, which results in a drive speed corresponding to the actual film speed.
Moreover, it is advantageous that a uniform driving torque is obtained because no springs have to be tensioned and relaxed over a stroke. In addition, higher driving speeds can be handled without any undesired oscillations of springs and masses.
Advantageous modifications and improvements of the device defined in the main claim can be obtained by means of the steps defined in the subsidiary claims.
A special advantage is that a tension-free film loop is situated before and after the carriage, as a result of which the film frame being scanned is free from its environment--with respect to tension, guidance--and is positioned only by the transport pins of the carriage. This enables the film frame to be moved freely by a few tenths of a millimetre in the x direction and the y direction and, in addition, to be rotated in the X/Y plane. The film loops should be dimensioned in such a manner that the reciprocating movement of the carriage is not obstructed.